Scientists dedicated to the search for dark matter, a mysterious substance that makes up 85% of the Universe's matter, reported identifying a singular particle interaction in the world's largest experiment in the field, conducted in the United States. This occurrence is not easily explained by current scientific knowledge.
According to one of the interpretations raised, this atypical energy signal could be the result of the collision between an atomic nucleus and a WIMP, a theoretical and heavy particle that serves as a possible candidate for dark matter. However, the team emphasized that this does not constitute a definitive discovery, but rather a promising starting point for investigating one of science's greatest enigmas.
Eric Dahl, a researcher at Northwestern University in the US and co-author of the study, commented: 'In the 20 years I have dedicated to the search for dark matter, this is the most interesting event I have ever seen.' If confirmed as a direct detection of dark matter, it would imply that it is even more enigmatic than previously thought.
What is dark matter?
It has been known for decades that most of the cosmic matter is composed of a type of invisible and almost undetectable matter, whose existence is inferred solely from its gravitational effects on other celestial bodies.
A milestone in this understanding occurred in the 1970s when American astronomer Vera Rubin noticed a peculiar phenomenon: stars at the edges of galaxies orbited at speeds comparable to those of central stars. This contradicted expected physical laws, as orbital speed should decrease when moving away from the center, similar to our Solar System where Mercury orbits much faster than Neptune.
Rubin concluded that the high speed of peripheral stars suggested they might be ejected from the galaxy unless there was invisible mass exerting enough gravity to keep it cohesive. Her measurements are considered primary evidence for dark matter. Currently, it is estimated that 84% of the Universe's matter is dark matter, while only 16% is common, visible matter (such as planets, stars, and human beings).
Although the gravity of this matter has been detected, its composition remains unknown. Scientists speculate that it may be composed of novel particles, just as common matter is made of indivisible particles like quarks. One of the most accepted hypotheses posits that dark matter is formed by WIMPs, an abbreviation in English for 'weakly interacting massive particle.'
These theoretical particles would be extremely heavy—with a mass at least one hundred times greater than a proton—but would only interact through gravity and the faint weak nuclear force, one of the fundamental forces of the Universe. Their lack of interaction with electromagnetic radiation (light, radio waves, infrared, etc.) makes their location extremely difficult.
Have we found dark matter?
The LUX–ZEPLIN, or LZ, experiment, located 1.5 kilometers deep in an old gold mine in South Dakota, USA, is the main instrument in this hunt for WIMPs. This complex apparatus is maintained by an international collaboration involving more than 30 research institutions.
The LZ operates with a giant reservoir containing ten tons of pure liquid xenon. This noble gas turns into a liquid at temperatures of -108 °C. The premise of the LZ is simple: if WIMPs are present on Earth, one of them will eventually collide with a xenon atom in the tank, generating a measurable energy signal.
Since 2021, the experiment monitored signals aligned with the most widespread theoretical model of WIMPs, where the particle would move slowly and its impact on the xenon nucleus would produce a low-energy signal. Initially, the search was unsuccessful. Recently, scientists expanded the analysis to include signals compatible with other WIMP models, originating from more elaborate theories that predicted higher-energy signals.
It was in this context that an anomalous event was recorded. On June 16, 2023, something triggered a considerable energy signal in the LZ tank, the origin of which scientists cannot determine. One possibility is that this event is caused by a distinct WIMP, more massive and energetic than predicted in the standard model, aligned with more complex physical theories, although not necessarily with the best-known model.
However, there is no official confirmation; researchers fear that the signal could be the result of contamination or background noise. The team spent months calculating other possibilities and found no clear explanation, estimating that the chance of the event being merely a statistical fluctuation or background noise is only 0.5%. The findings were presented at a conference in Japan in early September, and the paper was made available as a preprint, awaiting peer review. The team plans to submit the study to the journal Physical Review Letters.
Rick Gaitskell, a physics professor at Brown University in the US and head of the experiment, stressed: 'With only one event, we do not want to rush. We are not claiming to have detected dark matter. However, we observed something interesting that we want to share with the scientific community to get their input.'
The scientific community remains enthusiastic but cautious, given the historical skepticism in the field, where previous claims of dark matter detection turned out to be calculation errors or interference. The positive prospect lies in the fact that, if the data truly comes from dark matter, it must be repeatable. Scientists used only a small portion of the data collected by LZ, which will remain operational for years. Additionally, there are other global projects, such as PandaX-xT, which is under construction in China and promises to be the most powerful WIMP detector in the world.